The present invention relates to an optical amplifier in which Raman amplification is adopted and an optical multiplex transmission system using the same.
A conventional optical transmission system having an optical amplifier is shown in FIG. 1. Repeaters 1 and 2 acting as optical repeaters are interconnected with an optical transmission line 3. In respective repeaters 1 and 2, optically multiplexed main signal light is amplified by optical amplifiers 10 and 20 to be incident into optical transmission line 3.
The main signal light passing through optical transmission line 3 is forwarded to repeater 1 and 2 to be amplified by optical amplifiers 11 and 21.
In regard to the signal transmission from repeater 1 to repeater 2, an optical supervisory channel (OSC) signal is wavelength-multiplexed or superposed with a main signal to transmit to repeater 2. In repeater 2, wavelength-multiplexed signal light input through optical transmission line 3 is amplified to the sufficient optical level to establish the communication.
A multiple of signal light is wavelength-multiplexed in optical multiplex communication, which necessitates great optical power for the transmission of the multiplexed main signals. Therefore, if a failure on optical transmission line 3 (such as disconnection of an optical connector, optical fiber cutoff, etc.) occurs, and if the optical power leaks out of optical transmission line 3 to be directly irradiated to the human body, it is possible to injure the human body.
Recent development of communication infrastructure has brought about demands for larger capacity communication to longer distance. In order to realize long distance transmission, there has been increasing amplification systems utilizing Raman effect in optical fiber transmission lines.
In FIG. 2, there is shown an example of an optical transmission system by the use of Raman amplification. Raman amplification is a method of obtaining Raman effect by irradiating excitation light (Raman light) produced by Raman excitation light source 22, 12 to the fiber transmission line. It is necessary to raise the level of Raman light to input directly into optical transmission line 3 so that sufficient Raman gain may be obtained.
Accordingly, in optical transmission line 3, optical multiplex transmission using Raman amplification requires more light power than ordinary optical multiplex communication, which may cause serious injury to the human body.
Therefore, it is necessary to adjust light output in case a failure occurs in optical fiber transmission line 3.
It is therefore an object of the present invention to provide an optical amplifier which enables to control light output in the occurrence of fiber failure, and to provide an optical multiplex transmission system using the optical amplifier.
To attain the above-mentioned object of the present invention, the optical amplifier using Raman amplification is provided with a Raman excitation light source for generating Raman light and a detection circuit for detecting an error condition of an OSC signal being transferred with an optical transmission signal. When the detection circuit detects a bit error condition, the optical amplifier controls Raman light output power depending on the state of the bit error condition.
As one aspect of the present invention, preferably there is provided a circuit in the optical amplifier for informing a downstream optical amplifier of a transmission line failure using a flag superposed on an OSC signal for indicating an OSC circuit condition when the detection circuit detects an error becoming a predetermined condition in the OSC signal.
As another aspect of the present invention, preferably an optical amplifier using Raman amplification supervises a flag indicating OSC line condition in each OSC signal received from an upper stream of a transmission line concerned and from an upper stream of a transmission line in the opposite direction.
As still another aspect of the present invention, preferably the detection circuit supervises a signal error on the OSC signal to remove the light output control against Raman excitation light source when the signal error becomes a predetermined condition.
As still further aspect of the present invention, preferably the detection circuit outputs Raman light which produces Raman gain corresponding to an OSC signal bandwidth onto the transmission line fiber when Raman light output control against the Raman excitation light source is removed. The detection circuit then awaits the restoration of the OSC line. After the OSC signal is restored to the normal operation, Raman light is output to produce Raman gain in the wavelength required for the system according to wavelength information indicated through the OSC line.
Further scopes and features of the present invention will become more apparent by the following description of the embodiments with the accompanied drawings.